Now that we’re a few weeks to a month into the growing season, I’ve begun to turn my attention to the process of shoot thinning. The tender young shoots are beginning to harden off some and are ready to be thinned back to only what was selected at pruning.
This process, also known as suckering, is one of the most important steps in shaping the vine and adjusting the crop load for the year. Grape vines have a tendency to push many extraneous shoots from miscellaneous points on the plant in addition to those growing points that were selected at pruning. These shoots are known as “suckers” or “water shoots” when they emerge from the trunk or woody section of the vine and as “doubles” when they push in conjunction with primary shoots that were selected at pruning. The process of suckering is one by which all these secondary shoots are removed and the vine is stripped back to the 4 to 5 shoots per foot of cordon that is ideal for proper vine growth.
The visual difference between an un-thinned and a thinned vine can be dramatic, with sometimes nearly half the shoots on the plant being dropped to the ground, leaving what looks like a very skeletal vine. The importance of this process, however, can’t be stressed highly enough. First of all, suckers and doubles tend to be less fertile than the primary shoots to which we pruned. They bear little crop and serve only to crowd the canopy of the vine and sap strength from the primary shoots. Secondly, a crowded canopy—the green growing portion of the vine that is trained into the wires of the trellis—is an ideal environment for disease. Insofar as the Sunstone Estate is an organic vineyard, we rely on cultural practices such as shoot thinning to limit disease pressure and reduce our dependence on sulfur and horticultural oil for mildew control. These products must be used in limited quantities to prevent the growth of powdery mildew and are benign, organically acceptable means of disease control, but cultural methods such as suckering are our primary line of defense against disease issues.
Certain varieties require more intense suckering than others. Viognier is perhaps the most difficult variety to shoot thin, as it tends to push an overabundance of suckers and doubles—sometimes even triples. Merlot and Syrah can be very easy, as the secondary buds tend to be relatively unfertile. Cabernet Sauvignon and Cabernet Franc fall somewhere in the middle. Suckering proceeds in much the same order as does budbreak, with Viognier being followed with Merlot, Syrah, Cabernet Franc and, finally, Cabernet Sauvignon and Mourvedre.